1. Field of the Invention
The present invention relates to an optical pickup method, an optical pickup device, and an optical information processing apparatus.
2. Description of the Related Art
It is known that optical pickup devices record information in and reproduce information from an optical recording medium, such as CD (compact disk) or DVD (digital video disk), by irradiating it with light. Various systems have been proposed and put into practical use.
In recent years, various optical recording media have been put into practical use, for example, CD-ROM, DVD-ROM, WORM (write once, read many) media, or RW (rewritable) media. In addition, there is a type of optical recording medium that emits a fluorescent light in accordance with the amount of the light irradiated.
Moreover, a multi-layer optical recording medium has been proposed for practical use, which includes a plurality of recording layers provided in the single medium. See xe2x80x9cProgress In Optical Disk Recording With Over 20 GB Of Capacityxe2x80x9d in No.MB1 of ODS 2000 symposium.
When recording information in an optical recording medium, the medium is irradiated with a light beam along the track of the medium. In order to perform the recording, the reproducing or the erasing for the optical recording medium, the light beam must be focused on the recording surface of the medium to form a beam spot thereon, and the beam spot must be accurately positioned on the rotating medium along the track thereof.
The focusing control, such as the knife-edge method or the astigmatism method, is carried out to focus the light beam on the recording surface of the medium so that beam spots are formed thereon in the focal conditions. In addition, the tracking control, such as the push-pull method, is performed to keep the beam spots on the recording surface in the focal conditions.
The sub-spot tracking control is a kind of the tracking control to keep the beam spots on the recording surface in the focal conditions. In the sub-spot tracking control, a light beam, emitted by the light source, is separated by a diffraction grating into three diffracted beams: the 0th order diffracted beam (m=0, where m indicates the order of diffraction) and the 1st order diffracted beams (m=xc2x11). A main spot is formed on the recording surface by the 0th order diffracted beam, and a pair of sub-spots, interposing the main spot between them, are formed on the recording surface by the 1st order diffracted beams. To perform the sub-spot tracking control, a tracking error signal is generated based on the quantities of light of the reflection beams received from the sub-spots of the medium.
Moreover, by using two diffraction gratings, an improved sub-spot tracking control may be performed. In the improved sub-spot tracking control, a light beam, emitted by the light source, is separated by the diffraction gratings into five diffracted beams: the 0th order diffracted beam (m=0) and the 1st order diffracted beams (m=xc2x11). A main spot is formed on the recording surface by the 0th order diffracted beam, and two pairs of sub-spots, interposing the main spot between them, are formed on the recording surface by the 1st order diffracted beams. To perform the improved sub-spot tracking control, a tracking error signal is generated based on the quantities of light of the reflection beams received from the four sub-spots of the medium. See, for example, Japanese Laid-Open Patent Application No.5-12700.
The optical recording media generally are configured with a transparent substrate and a recording layer (the recording surface) provided on the substrate. The light beam, emitted by the light source, is converted by the objective lens into a converging beam, and this converging beam is passed through the transparent substrate of the medium and focused on the recording surface of the medium.
According to the standards of optical recording disks, the thickness of the substrate of the optical recording media are specified depending on the type of the media. For example, the substrate of the CD type media is specified as being 1.2 mm thick, and the substrate of the DVD type media is specified as being 0.6 mm thick. However, among the products of optical recording media that are commercially available, the substrate thickness of the media generally deviates from the specified thickness due to the manufacturing errors.
The optical systems of the optical pickup devices are designed based on the specified thickness of the substrate of the medium being accessed. If the thickness of the substrate of the actually used medium significantly deviates from the specified thickness, the spherical aberration takes place due to the deviation of the substrate thickness of the medium. When the spherical aberration becomes large, the accuracy of the sub-spot tracking control will be lowered. In the conditions of the large spherical aberration, it is difficult to accurately focus the light beams onto the recording surface of the medium. It is difficult to irradiate the medium with the beam spots so as to correctly create a mark or pit on the recording surface of the medium. In such conditions, the quantities of light of the reflection beams received from the medium will be lowered, which causes the lowering of the jitter or the signal-to-noise ratio, or the deterioration of the quality of the reproduced information.
In order to eliminate the problem of the spherical aberration, a method of correcting the spherical aberration, due to the deviation of the substrate thickness of the optical recording medium, by changing a pattern of the light beams incident to the objective lens has been proposed as in Japanese Laid-Open Patent Application No.2000-30290.
In a case of an optical pickup device designed to convert the emitted light beam from the light source into a parallel light beam and make it incident to the objective lens, when the substrate thickness of the medium is larger than the specified thickness according to the standards, the focal position of the beam spot, formed by the beam from the objective lens, deviates from the recording surface of the medium toward the light source. To correct this error, the pattern of the light beam incident to the objective lens is changed to a slightly divergent light beam. By passing the divergent light beam through the objective lens, the spherical aberration due to the objective lens and the spherical aberration due to the deviation of the substrate thickness of the medium can be canceled each other. By this method, the beam spots with a proper size can be positioned on the recording surface of the medium in the focal conditions.
The spherical aberration correction method disclosed in Japanese Laid-Open Patent Application No.2000-30290 is effective in correcting the spherical aberration due to the deviation of the substrate thickness of the medium. However, in the case of the optical pickup device in which the sub-spot tracking control is performed, the positions of the sub-spots on the recording surface of the medium are shifted when the spherical aberration correction is performed. The tracking error signal that is produced based on the quantities of light of the reflection beams from the sub-spots of the medium is influenced by the shifting of the sub-spot positions, which causes the deterioration of the accuracy of the sub-spot tracking control.
In addition, when the multi-layer optical recording medium described above is accessed by the optical pickup device in which the sub-spot tracking control is performed, the positions of the sub-spots on the recording surface of the medium are shifted by not only the performance of the spherical aberration correction but also the location of the given one of the recording layers of the multi-layer medium from the medium surface. Also, the spherical aberration is varied depending on the location of the given one of the recording layers of the multi-layer medium from the medium surface. The tracking error signal that is produced based on the quantities of light of the reflection beams from the sub-spots of the medium is influenced by the shifting of the sub-spot positions, which causes the deterioration of the accuracy of the sub-spot tracking control.
An object of the present invention is to provide an improved optical pickup method in which the above-described problems are eliminated.
Another object of the present invention is to provide an optical pickup method which effectively prevents the deterioration of the accuracy of the sub-spot tracking control when the spherical aberration correction is performed to correct the spherical aberration due to the deviation of the substrate thickness of the medium.
Another object of the present invention is to provide an optical pickup device which effectively prevents the deterioration of the accuracy of the sub-spot tracking control when the spherical aberration correction is performed to correct the spherical aberration of the optical recording medium due to the deviation of the substrate thickness of the medium.
Another object of the present invention is to provide an optical information processing apparatus which effectively prevents the deterioration of the accuracy of the sub-spot tracking control when the spherical aberration correction is performed to correct the spherical aberration of the optical recording medium due to the deviation of the substrate thickness of the medium.
The above-mentioned objects of the present invention are achieved by an optical pickup method for accessing an optical recording medium, the medium including a transparent substrate and a recording surface on the substrate, the optical pickup method comprising the steps of: passing a light beam, emitted by a light source, through a grating unit to separate the emitted light beam into a 0th order diffracted beam and 1st order diffracted beams; passing the diffracted beams, sent from the grating unit, through an objective lens to focus the beams onto the recording surface of the medium through the substrate, so that a main spot is formed on the recording surface by the 0th order diffracted beam and sub-spots, interposing the main spot therebetween, are formed on the recording surface by the 1st order diffracted beams; receiving respective reflection beams from the main spot and the sub-spots of the medium to generate detection signals from the received reflection beams; changing a pattern of the beams incident to the objective lens to correct a spherical aberration due to a deviation of a thickness of the substrate of the medium; and moving the grating unit relative to the light source to cancel shifting of sub-spot positions on the recording surface due to the spherical aberration correction, in order to generate a proper tracking error signal.
The above-mentioned objects of the present invention are achieved by an optical pickup device for accessing an optical recording medium, the medium including a transparent substrate and a recording surface on the substrate, the optical pickup device comprising: a light source which emits a light beam; a grating unit which separates the light beam, emitted by the light source, into a 0th order diffracted beam and 1st order diffracted beams; an objective lens which focuses the diffracted beams, sent from the grating unit, onto the recording surface of the medium through the substrate, so that a main spot is formed on the recording surface by the 0th order diffracted beam and sub-spots, interposing the main spot therebetween, are formed on the recording surface by the 1st order diffracted beams; a reflection beam detector which receives reflection beams from the main spot and the sub-spots of the medium to generate detection signals from the received reflection beams; and a control unit which changes a pattern of the beams incident to the objective lens to correct a spherical aberration due to a deviation of a thickness of the substrate of the medium, and the control unit moving the grating unit relative to the light source to cancel shifting of sub-spot positions on the recording surface due to the spherical aberration correction, in order to generate a proper tracking error signal.
The above-mentioned objects of the present invention are achieved by an optical information processing apparatus in which an optical pickup device is provided to access an optical recording medium, the medium including a transparent substrate and a recording surface on the substrate, the optical pickup device comprising: a light source which emits a light beam; a grating unit which separates the light beam, emitted by the light source, into a 0th order diffracted beam and 1st order diffracted beams; an objective lens which focuses the diffracted beams, sent from the grating unit, onto the recording surface of the medium through the substrate, so that a main spot is formed on the recording surface by the 0th order diffracted beam and sub-spots, interposing the main spot therebetween, are formed on the recording surface by the 1st order diffracted beams; a reflection beam detector which receives reflection beams from the main spot and the sub-spots of the medium to generate detection signals from the received reflection beams; and a control unit which changes a pattern of the beams incident to the objective lens to correct a spherical aberration due to a deviation of a thickness of the substrate of the medium, and the control unit moving the grating unit relative to the light source to cancel shifting of sub-spot positions on the recording surface due to the spherical aberration correction, in order to generate a proper tracking error signal.
According to the optical pickup method and device of the present invention, the shifting of the sub-spot positions due to the performance of the spherical aberration correction is corrected by movement of the grating unit relative to the light source, so that the sub-spots with a proper pitch are positioned on the recording surface of the medium. Therefore, the optical pickup method and device of the present invention are effective in preventing the deterioration of the accuracy of the sub-spot tracking control when the spherical aberration correction is performed.